The present invention relates to an electrochemical etching method and apparatus. In particular, the present invention relates to a method and apparatus in which an n-type silicon substrate is exposed at one surface to an electrolyte and at an opposite surface to light, so that a pore (hole) or a trench (groove) of a certain size and shape is formed in the substrate as an etching current flowing in the substrate is controlled by the light. Also, the present invention relates to a product, e.g., a semiconductor device, made by the use of the electrochemical etching method. It is to be understood that the present invention is preferably applicable to a method and apparatus for an electrochemical etching for the formation of pores or trenches having a diameter or width of 50 nm or more in the n-type silicon substrate. However, the present invention is not limited by the size of the pore or trench.
Japanese Patent Publication No. 2,694,731 discloses an electrochemical etching system which uses light to form small pores or trenches in an n-type doped silicon substrate. The system has a holder for holding the n-type doped silicon substrate (silicon wafer) with one surface of the substrate contacted with an electrolyte (hydrofluoricacid). Also, the holder retains an electrode in the electrolyte so that the electrode opposes the silicon substrate. With this etching system, the silicon substrate is positively biased and the electrode in the electrolyte is negatively biased. The opposite side of the silicon substrate away from the electrolyte is exposed to light, causing holes in he silicon substrate. The holes travel a boundary region between the silicon substrate and the electrolyte to resolve the boundary portion of the silicon substrate. This means that an arrangement of a masking barrier (coating) with one or more apertures (pits) on the surface of the silicon substrate adjacent to the electrolyte, results in the formation of the pores or trenches in the substrate portions, corresponding to the apertures.
The Journal of Electrochemical Society, No. 140, October 1993, pp. 2836-2843 discloses a back light device for the illumination of silicon substrate. The light device has a lamp for emitting light, an infrared filter for removing infrared light from the emitted light, and a convex lens for collimating the light emitted from the lamp.
Also, the Journal of Electrochemical Society, No. 137, February 1990, pp. 653-659 discloses an electrochemical etching device which uses a 100 W tungsten lamp for the back light device.
Further, Japanese Patent Publication No. 11-509644 discloses a system for manufacturing devices with electrochemical etching. Japanese Patent Publication No. 11-154737 discloses a manufacturing system for incorporating a capacitance in the trench formed by the electrochemical etching technique. The Journal of Electrochemical Society, No. 137, February 1990, pp. 653-659 discloses an embodiment in which an aperture or trench of 20 xc3x9720 mm is formed in the silicon substrate by the etching technique.
In order to mass-produce various devices using the electrochemical etching system, the system is required to make an even etching for the entire surface of a relatively large silicon substrate with a diameter of three inches or more, for example, and thereby form pores or trenches of a certain size and shape at every portion of the substrate.
Using the system disclosed in the Japanese Patent Publication No. 2,694,731 and the 100 W tungsten lamp in the Journal of Electrochemical Society, No. 140, trials were made to form pores of a certain diameter at every portion in the three-inch silicon substrate. The etched silicon substrate was viewed by the microscope, which showed that pores were formed only in a limited part of the silicon substrate. Also, the resultant pores have different sizes and shapes. Although further trials were made under different conditions in voltage, current and illumination, uniform pores failed to be formed over the entire portion of the silicon substrate.
Another object of the present invention is to provide devices, e.g., semiconductor device and sensors, such as acceleration sensor, manufactured through such electrochemical etching method.
In order to attain the objects, an electrochemical etching system according to one aspect of the present invention has an illumination unit including a light source for illuminating an illumination surface of an n-type silicon substrate with an illumination of 10 mW/cm2 or more. According to the embodiment, even for the silicon substrate having a diameter of three inches or more, pores and/or trenches to be formed in the silicon substrate develops toward the illumination surface with a uniform cross section. Also, the formed pore and/or trench has a smooth surface.
In another aspect of the electrochemical etching system of the present invention, on the illumination surface of the silicon substrate, a ratio of the maximum illumination to the minimum illumination is 1.69:1 or less. With the arrangement, even for the silicon substrate having a diameter of three inches or more, a constant etching current flows in the silicon substrate, which ensures that the formed pore and/or trench has a substantially constant size (cross section and depth).
In another aspect of the electrochemical etching system of the present invention, a reference electrode is positioned in the electrolyte. A voltage detector with an elevated impedance is electrically connected between the reference electrode and the n-type silicon substrate. With the arrangement, by controlling the voltage between the reference electrode and the silicon substrate, the voltage to be applied to the silicon substrate can be controlled.
In another aspect of the electrochemical etching system of the present invention, an illumination unit has an illumination controller for controlling an illumination to the other surface of the silicon substrate. With the arrangement, the size of the pore and/or trench to be formed in the silicon substrate can be controlled.
In another aspect of the electrochemical etching system of the present invention, the illumination controller controls an amount of light emitted from the light source. With the arrangement, the illumination to the silicon substrate can be adjusted precisely.
In another aspect of the electrochemical etching system of the present invention, the illumination controller, which is positioned between the light source and the silicon substrate, has a modulator for modulating light emitted from the light source. With the arrangement, where the light source is unable to control an amount of light to be emitted therefrom, the illumination of the silicon substrate can be controlled.
In another aspect of the electrochemical etching system of the present invention, the system includes a current detector for detecting a current applied from the power source to the silicon substrate, and a circuit for controlling the emitting light according to the current detected by the current detector. With the arrangement, the silicon substrate can be etched precisely.
In another aspect of the electrochemical etching system of the present invention, the system includes a unit for maintaining a stable condition of the hydrofluoricacid (e.g., concentration and temperature). With the arrangement, the hydrofluoricacid has a stable condition, which in turn ensures the constant size of the pore and/or trench formed in the silicon substrate.
In another aspect of the electrochemical etching system of the present invention, the system has a metal plate positioned on the other surface of the silicon substrate. The metal plate is formed with a number of regularly arranged openings so that light from the illumination unit toward the silicon substrate is transmitted therethrough. With the arrangement, the other surface of the silicon substrate is illuminated uniformly, which ensures the silicon substrate to be applied with a constant voltage.
In another aspect of the electrochemical etching system of the present invention, the metal plate is made of electrically conductive material and is positioned adjacent to the other surface of the silicon substrate. With the arrangement, the power source and the silicon substrate are electrically connected through the metal plate.
In another aspect of the electrochemical etching system of the present invention, the metal plate is formed integrally on the other surface of the silicon substrate. With the electrochemical etching system, the metal plate is formed precisely by the physical or chemical vapor deposition and also the lithography used in the manufacturing process for semiconductor. Also, the openings can be formed with a great precision.
In another aspect of the electrochemical etching system of the present invention, the metal plate is formed independent of the silicon substrate. With the arrangement, the manufacturing process of the silicon substrate can be simplified.
An electrochemical etching method of the present Invention which includes the steps of making one surface of an n-type silicon substrate into contact with an electrolyte, illuminating the other surface of the silicon substrate, and controlling an etching current by the illumination to form a pore or trench in the one surface of the silicon substrate is characterized in that the method further comprises illuminating the the other surface of the silicon substrate with an illumination of 10 mW/cm2 or more.
In another aspect of the electrochemical etching method, the method includes arranging a metal plate with a number of regularly arranged openings on the other surface of the n-type silicon substrate and illuminating the other surface of the n-type silicon substrate through the openings.
In another aspect of the electrochemical etching method, a ratio of a maximum illumination to a minimum illumination to the other surface of the silicon substrate is 1.69:1 or less.
With the methods, even the silicon substrate having a diameter of three inches or more is formed with a substantially the same size pores and/or trenches in an entire area of the substrate.
Another electrochemical etching method of the present invention which has the steps of making one surface of an n-type silicon substrate into contact with an electrolyte, illuminating the other surface of the silicon substrate, and controlling an etching current by the illumination to form pores or trenches in the one surface of the silicon substrate is characterized in that the method further comprises a first step in which the other surface of the silicon substrate is illuminated with a first illumination of 10 mW/cm2 or more to form the pores or trenches extending toward the other surface of the silicon substrate, and a second step in which, after the first step, the other surface of the silicon substrate is illuminated with another illumination higher than the first illumination to extend the pores or trenches laterally to connect the pores or trenches to each other. According the method, the vertical pores can be connected at bottom portion thereof to each other.
In view of above, according to the electrochemical etching method of the present invention, the shapes of the pores and/or trenches can be controlled so precisely. Also, an enlarged substrate can be etched. Then, the devices manufactured by the electrochemical etching system has pores and/or trenches of which shape is well controlled, ensuring a high performance and its inexpensiveness.